The invention relates to a method for wave-soldering component groups, especially printed circuit boards, with shielding from the environment, in which lead-free or at least low-lead solders, especially tin solders, with a melting point of more than 210.degree. C. are used. As used herein, the term "low-lead solder" refers to a solder containing a maximum of 10 wt-% lead.
Lead-rich tin/lead solders with a melting point of about 185.degree. C. are widely used today for soft-soldering electronic components. Especially in the case of wave-soldering processes, i.e., an operating principle in which the solder is transferred by a wave of molten solder onto the workpiece, the solder bath from which the wave is formed is set at a temperature that is 50.degree. to 60.degree. C. above the melting point of the solder. This temperature setting serves largely for the purpose of obtaining a solder of good viscosity which thereafter will form flawless and high-quality soldered junctions.
If in this process low-lead or lead-free solders of higher melting points are to be used--which is increasingly desirable today for environmental reasons--it will be necessary to establish higher solder bath temperatures. The lead-free or low-lead solders generally have a melting point that is at least about 30.degree. C. higher than the commonly used eutectic tin/lead solders which melt at about 183.degree. C. and are employed at a solder bath temperature of about 240.degree. C. (see, for example, the article, "Weichloten unter aktiviertem Schutzgas . . . " [soft soldering under activated protective gas . . . " ] in ELEKTRONIK PRODUKTION & PRUFTECHNIK--Apr. 1989, pp. 37-39, particularly p. 37, middle column). In contrast, the solders here addressed operate at solder bath temperatures of at least approximately 250.degree. C.
When solder bath temperatures are above 250.degree. C., however, there is a decidedly greater danger that electronic components might be heat-damaged. Consequently, there are components that cannot be soldered with lead-free or low-lead solders of high melting points, or that can be soldered only with a greater number of rejections for defects. For a discussion of temperature considerations in soldering see, for example, Volk, U.S. Pat. No. 5,259,546 (=DE 4,016,366) the disclosure of which is incorporated herein by reference.